This invention relates to an apparatus for measuring the thickness of plastic film made by the blown film process, wherein a plastic film bubble is created by extrusion of plastic through a circular die, and the bubble is subsequently collapsed in an "A" frame assembly, and fed through a series of rollers through one or more collection points.
Machines for producing plastic film by the blown film process operate generally on the principle of producing a plastic bubble by means of air interaction with molten plastic through a narrow annular die gap. The die gap is typically formed by the concentric positioning of two dies so as to provide an intermediate annular gap therebetween. The plastic bubble which is created by the machine is usually collapsed in the region above the die, by pulling the plastic film through a collapsing "A" frame, wherein a gap at the top of the frame permits the film to be passed through the frame and over a series of rollers. At least one set of these rollers is driven by a motor which has a controllable rotational speed, wherein the thickness of the plastic film may be controlled by varying the rotational speed of the motor, thereby varying the rotational speed of the rollers and the linear movement of the plastic film as it is collected. The film which is produced by this machine is then passed over a series of further rollers and edge cutters, and is ultimately collected on take-up reels for storage and disposition.
A continuing problem with machines of the type described herein is the problem of maintaining a film thickness within relatively narrow tolerances, and the further problem of correcting film thickness variations without excessive waste of film. Under optimum conditions it is desirable to adjust the correct thickness of the plastic film to be produced within a very short time after starting up the machine, i.e., within five to ten minutes. However, this requires that film thickness measurements be made very quickly in order for machine adjustments to be made, and in the prior art it has been very difficult to obtain a quick measurement of average film thickness. Plastic film as manufactured by these machines are typically made in thicknesses ranging from 0.001 to 0.010 inch (1-10 mils), and it is desirable to/control the thickness to .+-.3% or better. Film thickness is typically controlled by sensors which monitor the film as it is formed into the plastic bubble, and circuits which convert the sensor signals to motor drive signals to vary the speed of the haul-off rollers; i.e., the motor-driven rollers which pull the film through the collapsing frame. The haul-off rollers may be increased in speed in order to produce a thinner film, and decreased in speed in order to produce a thicker film.
A further problem with machines utilized in the blown film process, is the problem of localized imperfections or variations in the die, which produces localized regions of thicker or thinner film. Any die which utilizes an annular gap for producing film will invariably have variations in gap dimensions; a gap variation of only a few percent over a localized region is unacceptable, particularly when one considers that the film produced by the machine is collected on rollers, and a localized unevenness in the film will create a cumulative thickness as the take-up roller is filled. This leads to a lumpy and unstable roll of the finished product. This problem has been at least partially solved by rotating the die as the machine is operated, to essentially distribute imperfections about the circumference of the bubble, so that a buildup of thicker or thinner film material will not distort the take-up roll. In a typical machine, the die is rotated at a very slow rate, usually at about 3-10 minutes per revolution. Alternatively, some machines have mechanisms for rotating the "A" frame instead of the die. The blown film bubble moves at a linear speed in the range of 50-300 feet per minute, and therefore an imperfection in film thickness caused by the die gap will be distributed more or less evenly across the plastic film sheet which is ultimately placed on a take-up roll.
In compensating for film thickness variations, a thickness sensor placed against the outer surface of the bubble can provide an instantaneous measurement of thickness at its localized position. By virtue of rotation of the die, a thickness sensor will eventually measure the film thickness of the entire circumference of the bubble, because the film produced by the die will eventually move past the sensor. However, since the die is rotating at a relatively slow angular rate, a film thickness measurement of the entire circumference will require some 3-10 minutes of operation, during which time 100-3,000 feet of film may have been manufactured. If the overall average film thickness is measured, and is found to be outside permissible limits, the rotational speed of the haul-off rollers can be controlled, but at the cost of producing a considerable amount of film outside of specifications. In the prior art, this problem has been addressed by mounting a sensor for measuring thickness on a circular trolley, and transporting the sensor about the bubble at the same time as the bubble is being formed. The rate of rotation of the sensor about the bubble is typically controlled at 2-6 minutes per revolution, thereby permitting a complete measurement of thickness of the entire bubble to be made within a much shorter period of time. While this does reduce the response time required by the system for correcting film thickness errors, it requires the additional expense of constructing the circular trolley system for the sensor. Further, since the relative movement of both sensor and the plastic film bubble occurs while the sensor measurements are being made, measurement errors may be caused by the failure of the sensor to maintain close contact with the film bubble.
In systems of the foregoing type, the optimum placement of a circular trolley for holding a sensor is at a position between the die and the collapsing frame, where the bubble is still generally circular in shape. The temperature of the bubble in this region is much higher than the bubble temperatures found at a greater distance from the die, and particularly is much greater than the temperatures of the bubble in the collapsing frame. Therefore, temperature variations create additional problems, requiring that the thickness sensor be capable of operating at a higher temperature, and increasing the accumulation of additives and waxes which necessarily form during the process. These materials are by-products of the blown film production, and have a tendency to fill the gaps in the thickness sensor, to thereby distort or destroy the sensor readings. In addition, the temperature of the bubble varies considerably at different points about the bubble circumference, and therefore the sensor must be capable of producing stable thickness measurements around the circumference of the bubble in spite of temperature variations around the circumference.